Present day state-of-the-art design technique, logic synthesis, is really only a mapping between different levels of physical abstraction.
One of the most difficult problems in design automation is the inability to get timing closure at even the gate level effectively. This forces designers to do two designs: logic design and timing design. Otherwise, the designer simply over-designs the circuits, because the best case timing is much different from the worst case timing. In other cases, designers insist on control of device layout so that they can evaluate all of the tradeoffs between implementation and timing.
Present computer aided design (CAD) systems for the design of electronic circuits, referred to as ECAD or Electronic CAD systems, assist in the design of electronic circuits by providing a user with a set of software tools running on a digital computer with a graphical display device. Typically, five major software program functions run on the ECAD system: a schematic editor, a logic compiler, a logic simulator, a logic verifier, and a layout program. The schematic editor program allows the user of the system to enter and/or modify a schematic diagram using the display screen, generating a net list (summary of connections between components) in the process. The logic compiler takes the net list as an input, and using a component database puts all of the information necessary for layout, verification and simulation into a schematic object file or files whose format(s) is(are) optimized specifically for those functions. The logic verifier checks the schematic for design errors, such as multiple outputs connected together, overloaded signal paths, etc., and generates error indications if any such design problems exist. The logic simulator takes the schematic object file(s) and simulation models, and generates a set of simulation results, acting on instructions initial conditions and input signal values provided to it either in the form of a file or user input. The layout program generates data from which a semiconductor chip (or a circuit board) may be laid out and produced.
The Modular Design Environment (MDE) produced by LSI Logic Corporation of Milpitas, California, is a suite of software tools for computers running the UNIX operating system. MDE comprises a schematic editor (LSED) and a simulator (LDS), among other software programs, and provides an example of commercially available tools of the aforementioned type. Another example of a schematic editor, schematic compiler, and schematic simulator may be found in the SCALDstation produced by Valid Logic Systems, Inc. of Mountain View, Calif.
VHDL, or VHSIC (Very High Speed Integrated Circuit) Hardware Description Language, is a recently developed, higher level language for describing complex devices. The form of a VHDL description is described by means of a context-free syntax together with context-dependent syntactic and semantic requirements expressed by narrative rules. VHDL is described in IEEE Standard VHDL Language Reference Manual (IEEE Std 1076-1987), and is also known as MIL-STD-454, Regulation 64.
VHDL represents an important step forward in design specification languages because the semantics, or intent, of the language constructs are clearly specified. In theory, VHDL unambiguously describes a designer's intended system or circuit behavior, in syntactic terms. The "design entity" is the primary hardware abstraction in VHDL. It represents a portion of a hardware design that has well-defined inputs and outputs and performs a well-defined function. A design entity may represent an entire system, a sub-system, a board, a chip, a macro-cell, a logic gate, or any level of abstraction in between. A "configuration" can be used to describe how design entities are put together to form a complete design.
VHDL supports three distinct styles for the description of hardware architectures. The first of these is "structural" description, wherein the architecture is expressed as a hierarchical arrangement of interconnected components. The second style is "data-flow" description, in which the architecture is broken down into a set of concurrent register assignments, each of which may be under the control of gating signals. This description subsumes the style of description embodied in register transfer level (RTL) descriptions. The third style is "behavioral" description, wherein the design is described in sequential program statements similar to a high-level programming language. In the main hereinafter, the behavioral description style is discussed. However, all three styles may be intermixed in a single architecture.
A methodology for deriving a lower-level, physically-implementable description, such as a RTL description of the higher level (e.g. VHDL) description, via an intermediate rule-based tool such as Prolog, is disclosed herein.
Prolog is a programming language based on predicate logic. It can be used for "intelligent" tasks like mathematical theorem proving. A Prolog program is a set of rules which define the relationships among objects. The general form of a Prolog rule is a "horn" clause, in which a specified "goal" is true if certain conditions are true. Execution of a Prolog program involves finding a proof for the goal in question, using unification and resolution. An important aspect of Prolog employed in the present invention is "term.sub.-- expansion", which converts predefined rules into ordinary Prolog clauses.